Liquid crystal on silicon (LCoS), LCD panel, and digital micromirror device (DMD) projectors project a light image by the modulation of a light source (or lamp) using the respective (LCoS, LCD, DMD) modulation device or modulation devices. To improve sequential contrast and gray-level bit depth, it is known to attenuate the light source of a projection system in accordance with the video signal. For example, when displaying a fully black screen between video segments, ideal infinite sequential contrast could be obtained by simply turning off the lamp. Likewise in dark scenes with brightness levels only a small fraction of maximum, the output of the lamp might be reduced to improve the sequential contrast. In theory, by attenuating the light source in this way, the full bit-depth of the modulation device (or panel) can be used with a scaled luminance level to provide an increased sequential contrast. In most practical systems, however, such as in a ultra-high-pressure (UHP) mercury lamp, the light source itself cannot be intrinsically diminished in a practical way, due to speed constraints and other known difficulties, and instead an external aperture has been used to control light output.
A known “aperture” method for reducing the intensity output of a light source is the use of an electromechanical auto-iris as a dynamic aperture stop to provide active attenuation of a projection light source. FIG. 1 shows such a known mechanical auto-iris illumination system 100, in which an electromechanical device provides an auto-iris 130 with a variable aperture, having an array of curved spring-steel leaves whose orientation is determined electro-mechanically by the relative position of two rings. Illumination system 100 attenuates light from light source 102 by mechanically increasing or decreasing the aperture 132 through which the illumination passes. Auto-iris 130 is used to attenuate light source 102 achromatically to increase dynamic range of the system 100, and also to reduce the working f-stop (f/#), increasing contrast at the lower intensity settings. Dynamic range in this context describes the ability to maintain pixel gray level count at various image intensity settings, which in turn means the display has a good full gray representation for bright and dark images. Closing the aperture 132 therefore both attenuates the illuminating light and increases the working f/#, the latter also helping with contrast at any given light level, though often at the expense of uniformity. Though quite effective, mechanical components such as auto-iris 130 within illumination system 100 are prone to reliability issues, are costly, noisy, provide undesirable vibration, and have relatively slow response times.